Many types of doors and hardware are used in hard traffic, commercial, institutional, and industrial door openings in buildings. When trying to achieve a specific function and design for an opening, the factors of aesthetics, security and resistance to abuse must be considered. Often when double doors are installed at a particular location, a necessary piece of hardware is a center mullion. Depending on the design and function of the opening, a mullion may be permanently fixed in place to secure the doors, or the mullion can be engineered so that it is removable, for example with a key. In this case the mullion is defined as a hardware mullion.
A mullion provides a vertical structure that two push bar panic devices can be locked into, and enables locking of a pair of doors from the outside. The push bars on the inside of the doors cannot be locked from the inside. They are always operable to open the door and exit a room, corridor or building when the bar is pushed. A keyed removable electric hardware mullion is a hollow rectangular or square, steel or aluminum post that allows an electric strike to be mounted onto one, or both, of the mullion faces.
In conventional doorways that include a mullion and a pair of doors, the pair of doors are closed and locked by the electric strikes, which capture the panic device latch bolts until they are released by a remote low voltage signal, such as a key switch, push button or through a card reader credential or keypad.
Mullions can be temporarily removed by key when a wider opening is needed. A mullion is held in place by an aluminum or steel header block mounted to the underside of a door frame header at the top of the mullion. The header block is fitted with an electrical cable connection that is easily disconnected when the mullion is removed. When installed, cables are connected at the top of the mullion, and the mullion is fitted around the block at the top. The bottom of the mullion is then swung into place and locked into the electrical mullion receptacle at the threshold.
Alternative, conventional, and electric types of mullion are locked in place with a lock at the top of the mullion. When installed, this mullion is placed over a sill block at the sill of the threshold and tilted into place and locked into a header block at the top of the mullion. However because the components of the sill blocks project up into the mullion above the threshold, they may become a trip hazard and a potential issue when the mullion is temporarily removed. The Americans with Disabilities Act disallows the use of a threshold thicker than ½″ in an employee entrance or accessible route.
Also, some mullions, especially electric mullions, that use electric strikes to secure an opening, are difficult to remove and reinstall especially when the locking mechanism and the cable connections are both at the top of the mullion.
Sometimes, in order to enable the installation and removability of a mullion, a threshold must have a notch or slot cut out of it so the sill block can be mounted to the floor and not on top of the threshold to facilitate the mullion's removability. The slotted threshold is exposed when the mullion needs to be removed and may also become a trip hazard and an issue.